Preemptive packet for maintaining contiguity in cyclic prioritized multiple access (CPMA) contention-free sessions

ABSTRACT

An access point transmits a preemptive peg packet when it has no data to transmit in order to maintain the contiguity of its transmission timing position with respect to the timing position of other contention-free sessions (CFS) transmitted by other access points in an existing, periodic sequence. The cyclic prioritized multiple access (CDMA) method establishes the transmission timing position of contention-free sessions (CFS) between overlapping first and second wireless LAN cells contending for the same medium. Each cell includes a respective plurality of member stations. If an access point has no traffic, it will transmit a short, preemptive pegging packet and reset its backoff timer. In this manner, no gaps longer than the distributed coordination function (DCF) Interframe Space (DIFS) are left idle. This prevents other stations from using DCF contention to seize the channel, until all access points have completed one contention-free session (CFS) per periodic cycle.

This application is a continuation of U.S. patent application Ser. No.11/777,736 filed on Jul. 13, 2007, (now U.S. Pat. No. 8,068,470) whichis a continuation of U.S. patent application Ser. No. 10/256,309 filedon Sep. 27, 2002, (now U.S. Pat. No. 7,245,605), which claims priorityfrom:

-   -   [1] U.S. Provisional Patent Application No. 60/330,930, filed        Nov. 2, 2001 entitled “HCF ACCESS MECHANISM: OBSS MITIGATION”;    -   [2] U.S. Provisional Patent Application No. 60/331,030, filed        Nov. 7, 2001 entitled “‘NEIGHBORHOOD’ CAPTURE IN CSMA/CA WLANS”;    -   [3] U.S. Provisional Patent Application No. 60/331,211, filed        Nov. 13, 2001 entitled “‘SHIELD’ PROTECTING HIGH PRIORITY        CHANNEL ACCESS ATTEMPTS”; and    -   [4] U.S. Provisional Patent Application No. 60/342,343, filed        Dec. 21, 2001, entitled “WIRELESS LANS AND ‘NEIGHBORHOOD        CAPTURE’,” all of the above cited applications are incorporated        herein by reference.

RELATED APPLICATIONS

This patent application is related to patent application Ser. No.09/985,257, filed Nov. 2, 2001, (now U.S. Pat. No. 7,095,754), which isincorporated by reference.

This patent application is also related to patent application Ser. No.10/187,132, filed Jun. 28, 2002, which is incorporated by reference.

This patent application is also related to patent application Ser. No.10/256,299, filed Sep. 27, 2002, (now U.S. Pat. No. 7,248,600), which isincorporated by reference.

This patent application is also related to patent application Ser. No.10/256,384, filed Sep. 27, 2002, (now U.S. Pat. No. 7,280,517) which isincorporated by reference.

This patent application is also related to patent application Ser. No.10/256,471, filed Sep. 27, 2002, (now U.S. Pat. No. 7,277,415), which isincorporated by reference.

This patent application is also related to patent application Ser. No.10/256,305, filed Sep. 27, 2002, (now U.S. Pat. No. 7,245,604) which isincorporated by reference.

This patent application is also related to patent application Ser. No.10/256,516, filed Sep. 27, 2002, (now U.S. Pat. No. 7,180,905) which isincorporated by reference.

FIELD OF THE INVENTION

The invention disclosed broadly relates to telecommunications methodsand more particularly relates to wireless cells that have overlappingstations contending for the same medium.

BACKGROUND OF THE INVENTION

Wireless Local Area Networks (WLANs)

Wireless local area networks (WLANs) generally operate at peak Speeds ofbetween 10 to 100 Mbps and have a typical range of 100 meters.Single-cell wireless LANs are suitable for small single-floor offices orstores. A station in a wireless LAN can be a personal computer, a barcode scanner, or other mobile or stationary device that uses a wirelessnetwork interface card (NIC) to make the connection over the RF link toother stations in the network. The single-cell wireless LAN providesconnectivity within radio range between wireless stations. An accesspoint allows connections via the backbone network to wired network-basedresources, such as servers. A single-cell wireless LAN can typicallysupport up to 25 users and still keep network access delays at anacceptable level. Multiple-cell wireless LANs provide greater range thandoes a single cell through the means of a set of access points and awired network backbone to interconnect a plurality of single-cell LANs.Multiple-cell wireless LANs can cover larger, multiple-floor buildings.A mobile laptop computer or data collector with a wireless networkinterface card (NIC) can roam within the coverage area while maintaininga live connection to the backbone network.

Wireless LAN specifications and standards include the IEEE 802.11Wireless LAN Standard and the HIPERLAN Type 1 and Type 2 Standards. TheIEEE 802.11 Wireless LAN Standard is published in three parts as IEEE802.11-1999, IEEE 802.11a-1999, and IEEE 802.11b-1999, which areavailable from the IEEE, Inc. web sitehttp://grouper.ieee.org/groups/802/11. An overview of the HIPERLAN Type1 principles of operation is provided in the publication HIPERLAN Type 1Standard, ETSI ETS 300 652, WA2 Dec. 1997. An overview of the HIPERLANType 2 principles of operation is provided in the Broadband Radio AccessNetwork's (BRAN) HIPERLAN Type 2: System Overview, ETSI TR 101 683V1.1.1 (2000-02), and a more detailed specification of its networkarchitecture is described in HIPERLAN Type 2, Data Link Control (DLC)Layer; Part 4. Extension for Home Environment, ETSI TS 101 761-4 V1.2.1(2000-12). A subset of wireless LANs is Wireless Personal Area Networks(PANs), of which the Bluetooth Standard is the best known. The BluetoothSpecial Interest Group, Specification Of The Bluetooth System, Version1.1, Feb. 22, 2001, describes the principles of Bluetooth deviceoperation and communication protocols.

The IEEE 802.11 Wireless LAN Standard defines at least two differentphysical (PHY) specifications and one common medium access control (MAC)specification. The IEEE 802.11(a) Standard is designed to operate inunlicensed portions of the radio spectrum, usually either in the 2.4 GHzIndustrial, Scientific, and Medical (ISM) band or the 5 GHzUnlicensed-National Information Infrastructure (U-NII) band. It usesorthogonal frequency division multiplexing (OFDM) to deliver up to 54Mbps data rates. The IEEE 802.11(b) Standard is designed for the 2.4 GHzISM band and uses direct sequence spread spectrum (DSSS) to deliver upto 11 Mbps data rates. The IEEE 802.11 Wireless LAN Standard describestwo major components, the mobile station and the fixed access point(AP). IEEE 802.11 networks can also have an independent configurationwhere the mobile stations communicate directly with one another, withoutsupport from a fixed access point.

A single-cell wireless LAN using the IEEE 802.11 Wireless LAN Standardis an Independent Basic Service Set (IBSS) network. An IBSS has anoptional backbone network and consists of at least two wirelessstations. A multiple-cell wireless LAN using the IEEE 802.11 WirelessLAN Standard is an Extended Service Set (ESS) network. An ESS satisfiesthe needs of large coverage networks of arbitrary size and complexity.

Each wireless station and access point in an IEEE 802.11 wireless LANimplements the MAC layer service, which provides the capability forwireless stations to exchange MAC frames. The MAC frame transmitsmanagement, control, or data between wireless stations and accesspoints. After a station forms the applicable MAC frame, the frame's bitsare passed to the Physical Layer for transmission.

Before transmitting a frame, the MAC layer must first gain access to thenetwork. Three interframe space (IFS) intervals defer an IEEE 802.11station's access to the medium and provide various levels of priority.Each interval defines the duration between the end of the last symbol ofthe previous frame and the beginning of the first symbol of the nextframe, The Short Interframe Space (SIFS) provides the highest prioritylevel by allowing some frames to access the medium before others, suchas an Acknowledgement (ACK) frame, a Clear-to-Send (CTS) frame, or asubsequent fragment burst of a previous data frame. These frames requireexpedited access to the network to minimize frame retransmissions.

The Priority Interframe Space (PIFS) is used for high-priority access tothe medium during the contention-free period. A point coordinator in theaccess point connected to the backbone network controls thepriority-based Point Coordination Function (PCF) to dictate whichstations in the cell can gain access to the medium. The pointcoordinator in the access point sends a contention-free poll frame to astation, granting the station permission to transmit a single frame toany destination. All other stations in the cell can only transmit duringa contention-free period if the point coordinator grants them access tothe medium. The end of the contention-free period is signaled by thecontention-free end frame sent by the point coordinator, which occurswhen time expires or when the point coordinator has no further frames totransmit and no stations to poll. The Priority interframe Space (PIFS)is also known as the PCF Interframe Space.

The distributed coordination function (DCF) Interframe Space (DIFS) isused for transmitting low-priority data frames during thecontention-based period. The DIFS spacing delays the transmission oflower-priority frames to occur later than the priority-basedtransmission frames. An Extended Interframe Space (EIFS) goes beyond thetime of a DIFS interval, as a waiting period when a bad receptionoccurs. The EIFS interval provides enough time for the receiving stationto send an acknowledgment (ACK) frame.

During the contention-based period, the distributed coordinationfunction (DCF) uses the Carrier-Sense Multiple Access With CollisionAvoidance (CSMA/CA) contention-based protocol, which is similar to IEEE8023 Ethernet. The CSMA/CA protocol minimizes the chance of collisionsbetween stations sharing the medium by waiting a random backoff intervalif the station's sensing mechanism indicates a busy medium. The periodof time a minimal interval following traffic on the medium is when thehighest probability of collisions occurs, especially where there is highutilization. Once the medium is idle, CSMA/CA protocol causes eachstation to delay its transmission by a random backoff time, therebyminimizing the chance it will collide with those from other stations.

The CSMA/CA protocol computes the random backoff time as the product ofa constant, the slot time times a pseudo-random number RN that has arange of values from zero to a collision window CW. The value of thecollision window for the first try to access the network is CW1, whichyields the first-try random backoff time. If the first try to access thenetwork by a station fails, then the CSMA/CA protocol computes a new CWby doubling the current value of CW as CW2=CW1 times 2. The value of thecollision window for the second try to access the network is CW2, whichyields the second-try random backoff time This process by the CSMA/CAprotocol of increasing the delay before transmission is called binaryexponential backoff. The reason for increasing CW is to minimizecollisions and maximize throughput for both low and high networkutilization. Where there is a low network utilization, stations are notforced to wait very long before transmitting their frame. On the firstor second attempt, a station will make a successful transmission.However, if the utilization of the network is high, the CSMA/CA protocoldelays stations for longer periods to avoid the chance of multiplestations transmitting at the same time. If the second try to access thenetwork fails, then the CSMA/CA protocol computes a new CW by againdoubling the current value of CW as CW3=CW1 times 4. The value of thecollision window for the third try to access the network is CW3, whichyields the third-try random backoff time. The value of CW increases torelatively high values after successive retransmissions under hightraffic loads. This provides greater transmission spacing betweenstations waiting to transmit.

Collision Avoidance Techniques

Four general collision avoidance approaches have emerged: [1] CarrierSense Multiple Access (CSMA) [see, F. Tobagi and L. Kleinrock, “PacketSwitching in Radio Channels: Part I—Carrier Sense Multiple Access Modelsand their Throughput Delay Characteristics,” IEEE Transactions onCommunications, Vol. 23, No. 12, pp. 1400-1416, 1975], [2] MultipleAccess Collision Avoidance (MACA) [see, P. Karn, “MACA—A New ChannelAccess Protocol for Wireless Ad-Hoc Networks,” Proceedings of theARRL/CRRL Amateur Radio Ninth Computer Networking Conference, pp.134-140, 1990.], [3] their combination CSMA/CA, and [4] collisionavoidance tree expansion.

CSMA allows access attempts after sensing the channel for activity.Still, simultaneous transmit attempts lead to collisions, thus renderingthe protocol unstable at high traffic loads. The protocol also suffersfrom the hidden terminal problem.

The latter problem was resolved by the Multiple Access CollisionAvoidance (MACA) protocol, which involves a three-way handshake. [P.Karn, supra.] The origin node sends a request to send (RTS) notice ofthe impending transmission. A response is returned by the destination ifthe RTS notice is received successfully, and the origin node proceedswith the transmission. This protocol also reduces the average delay ascollisions are detected upon transmission of merely a short message, theRTS. With the length of the packet included in the RTS and echoed in theclear-to-send (CTS) messages, hidden terminals can avoid colliding withthe transmitted message. However, this prevents the back-to-backre-transmission in case of unsuccessfully transmitted packets. Afive-way handshake Multiple Access Collision Avoidance (MACA) protocolprovides notification to competing sources of the successful terminationof the transmission. [See, V. Bharghavan, A. Demers, S. Shenker, and L.Zhang, “MACAW: A media access protocol for wireless LANs,” SIGCOMM '94,pp. 212-225, ACM, 1994.]

CSMA and MACA are combined in CSMA/CA, which is MACA with carriersensing, to give better performance at high loads. A four-way handshakeis employed in the basic contention-based access protocol used in theDistributed Coordination Function (DCF) of the IEEE 802.11 Standard forWireless LANs. [See, IEEE Standards Department, D3, “Wireless MediumAccess Control and Physical Layer WG,” IEEE Draft Standard P802.11Wireless LAN, January 1996.]

Collisions can be avoided by splitting the contending terminals before,transmission is attempted. In the pseudo-Bayesian control method, eachterminal determines whether it has permission to transmit using a randomnumber generator and a permission probability “p” that depends on theestimated backlog. [See, R. L. Rivest, “Network control by BayesianBroadcast,” IEEE Trans. Inform. Theory, Vol. IT 25, pp. 505-515,September 1979.]

To resolve collisions, subsequent transmission attempts are typicallystaggered randomly in time using the following two approaches: binarytree and binary exponential backoff.

Upon collision, the binary tree method requires the contending nodes toself-partition into two groups with specified probabilities. Thisprocess is repeated with each new collision. The order in whichcontending nodes transmit is determined either by serial or parallelresolution of the tree. [See, J. L. Massey, “Collision-ResolutionAlgorithms and Random-Access Communications,” in Multi-UserCommunication Systems, G. Longo (ed.), CISM Courses and Lectures No.265, New York: Springer 1982, pp. 73-137.]

In the binary exponential backoff approach, a backoff counter tracks thenumber of pauses and hence the number of completed transmissions beforea node with pending packets attempts to seize the channel. A contendingnode initializes its backoff counter by drawing a random value, giventhe backoff window size. Each time the channel is found idle, thebackoff counter is decreased and transmission is attempted uponexpiration of the backoff counter. The window size is doubled every timea collision occurs, and the backoff countdown starts again. [See, A.Tanenbaum, Computer Networks, 3^(rd) ed., Upper Saddle River, N.J.,Prentice Hall, 1996.] The Distributed Coordination Function (DCF) of theIEEE 802.11 Standard for Wireless LANs employs a variant of thiscontention-resolution scheme: a truncated binary exponential backoff,starting at a specified window and allowing up to a maximum backoffrange below which transmission is attempted. [IEEE Standards Department,D3, supra.] Different backoff counters may be maintained by a contendingnode for traffic to specific destinations.

[Bharghavan, supra.]

In the IEEE 802.11 Standard, the channel is shared by a centralizedaccess protocol, the Point Coordination Function (PCF), which providescontention-free transfer based on a polling scheme controlled by theaccess point (AP) of a basic service set (BSS). [IEEE StandardsDepartment, D3, supra.] The centralized access protocol gains control ofthe channel and maintains control for the entire contention-free periodby waiting a shorter time between transmissions than the stations usingthe Distributed Coordination Function (DCF) access procedure. Followingthe end of the contention-free period, the DCF access procedure begins,with each station contending for access using the CSMA/CA method.

The 802.11 MAC Layer provides both contention and contention-free accessto the shared wireless medium. The MAC Layer uses various MAC frametypes to implement its functions of MAC management, control, and datatransmission. Each station and access point on an 802.11 wireless LANimplements the MAC Layer service, which enables stations to exchangepackets. The results of sensing the channel to determine whether themedium is busy or idle are sent to the MAC coordination function of thestation. The MAC coordination also carries out a virtual carrier senseprotocol based on reservation information found in the Duration Field ofall frames. This information announces to all other stations the sendingstation's impending use of the medium. The MAC coordination monitors theDuration Field in all MAC frames and places this information in thestation's Network Allocation Vector (NAV) if the value is greater thanthe current NAV value. The NAV operates similarly to a timer, startingwith a value equal to the Duration Field of the last frame transmissionsensed on the medium and counting down to zero. After the NAV reacheszero, the station can transmit if its physical sensing of the channelindicates a clear channel.

At the beginning of a contention-free period, the access point sensesthe medium; and if it is idle, it sends a beacon packet to all stations.The beacon packet contains the length of the contention-free interval.The MAC coordination in each member station places the length of thecontention-free interval in the station's Network Allocation Vector(NAV), which prevents the station from taking control of the mediumuntil the end of the contention-free period. During the contention-freeperiod, the access point can send a polling message to a member station,enabling it to send a data packet to any other station in the BSSwireless cell.

Quality Of Service (QoS)

Quality of service (QoS) is a measure of service quality provided to acustomer. The primary measures of QoS are message loss, message delay,and network availability. Voice and video applications have the mostrigorous delay and loss requirements. Interactive data applications suchas Web browsing have less restrained delay and loss requirements, butthey are sensitive to errors. Non-real-time applications such as filetransfer, email, and data backup operate acceptably across a wide rangeof loss rates and delay. Some applications require a minimum amount ofcapacity to operate at all—for example, voice and video. Many networkproviders guarantee specific QoS and capacity levels through the use ofService-Level Agreements (SLAs). An SLA is a contract between anenterprise user and a network provider that specifies the capacity to beprovided between points in the network that must be delivered with aspecified QoS. If the network provider fails to meet the terms of theSLA, then the user may be entitled a refund. The SLA is typicallyoffered by network providers for private line, frame relay, ATM, orInternet networks employed by enterprises.

The transmission of time-sensitive and data application traffic over apacket network imposes requirements on the delay or delay jitter, andthe error rates realized; these parameters are referred to genericallyas the QoS (Quality of Service) parameters. Prioritized packetscheduling, preferential packet dropping, and bandwidth allocation areamong the techniques available at the various nodes of the network,including access points, that enable packets from different applicationsto be treated differently, helping achieve the different quality ofservice objectives. Such techniques exist in centralized and distributedvariations.

Management of contention for the shared transmission medium must reflectthe goals sought for the performance of the overall system. Forinstance, one such goal would be the maximization of goodput (the amountof good data transmitted as a fraction of the channel capacity) for theentire system, or of the utilization efficiency of the RF spectrum;another is the minimization of the worst-case delay. As multiple typesof traffic with different performance requirements are combined intopacket streams that compete for the same transmission medium, amulti-objective optimization is required.

Ideally, one would want a multiple-access protocol that is capable ofeffecting packet transmission scheduling as close to the optimalscheduling as possible, but with distributed control. Distributedcontrol implies both some knowledge of the attributes of the competingpacket sources and limited control mechanisms.

To apply any scheduling algorithm in random multiple access, a mechanismmust exist that imposes an order in which packets will seize the medium.For distributed control, this ordering must be achieved independently,without any prompting or coordination from a control node. Only if thereis a reasonable likelihood that packet transmissions will be orderedaccording to the scheduling algorithm can one expect that thealgorithm's proclaimed objective will be attained.

The above-cited, copending patent application by Mathilde Benveniste,entitled “Tiered Contention Multiple Access (TCMA): A Method forPriority-Based Shared Channel Access”, describes the Tiered ContentionMultiple Access (TCMA) distributed medium access protocol that schedulestransmission of different types of traffic based on their QoS servicequality specifications. This protocol makes changes to the contentionwindow following the transmission of a frame, and therefore is alsocalled Extended-DCF (E-DCF). During the contention window, the variousstations on the network contend for access to the network. To avoidcollisions, the MAC protocol requires that each station first wait for arandomly chosen time period, called an arbitration time. Since thisperiod is chosen at random by each station, there is less likelihood ofcollisions between stations. TCMA uses the contention window to givehigher priority to some stations than to others. Assigning a shortcontention window to those stations that should have higher priorityensures that, in most cases, the higher-priority stations will be ableto transmit ahead of the lower-priority stations. TCMA schedulestransmission of different types of traffic based on their QoS servicequality specifications. A station cannot engage in backoff countdownuntil the completion of an idle period of length equal to itsarbitration time.

The above-cited, copending patent application by Mathilde Benvenistealso applies TCMA to the use of the wireless access point as a trafficdirector. This application of the TCMA protocol is called the hybridcoordination function (HCF). In HCF, the access point uses a pollingtechnique as the traffic control mechanism. The access point sendspolling packets to a succession of stations on the network. Theindividual stations can reply to the poll with a packet that containsnot only the response, but also any data that needs to be transmitted.Each station must wait to be polled. The access point establishes apolling priority based on the QoS priority of each station.

What is needed in the prior art is a way to reduce interference betweenoverlapping first and second wireless LAN cells contending for the samemedium.

SUMMARY OF THE INVENTION

In accordance with the invention, an access point transmits a preemptivepeg packet when it has no data to transmit in order to maintain thecontiguity of its transmission timing position with respect to thetiming position of other contention-free sessions (CFS) transmitted byother access points in an existing, periodic sequence. The cyclicprioritized multiple access (CPMA) method establishes the transmissiontiming position of contention-free sessions (CFS) between overlappingfirst and second wireless LAN cells contending for the same medium. Eachcell includes a respective plurality of member stations. If an accesspoint has no traffic, it will transmit a short, preemptive peggingpacket and reset its backoff timer. In this manner, no gaps longer thanthe distributed coordination function (DCF) Interframe Space (DIFS) areleft idle. This prevents other stations from using DCF contention toseize the channel, until all access points have completed onecontention-fire session (CFS) per periodic cycle.

DESCRIPTION OF THE FIGURES

FIGS. 1 through 1I show the interaction of two wireless LAN cells whichhave overlapping access points contending for the same medium, inaccordance with the invention.

FIG. 2A shows the IEEE 802.11 packet structure for a Shield packet, inaccordance with the invention.

FIG. 2B shows the IEEE 802.11 packet structure for a beacon packet,including the increment to the NAV period and the CFTR period.

FIG. 3 illustrates a timing diagram for the transmission of the shieldpacket.

FIG. 4 shows a timing diagram of a sample contention-free session (CFS)structure, which includes the shield packet, the beacon packet, and theexchange of data packets during the contention-free period shown inFIGS. 1, 1A through 1C.

FIG. 5 shows a timing diagram of non-conflicting contention-freesessions (CFS) for access point 152 (AP1) and access point 102 (AP2).

FIG. 6 shows a timing diagram of how access point 102 (AP2) listens fora PIFS idle following a busy channel and then starts transmitting aminimal interval after the contention-free session (CFS) for accesspoint 152 (AP1).

FIG. 7 shows a timing diagram of the successful startup of access point102 (AP2) after the contention-free session (CFS) for access point 152(AP1).

FIG. 8 shows a timing diagram of access point 102 (AP2) transmitting apreemptive peg packet when it has no data to transmit in order tomaintain contiguity of its timing position in the periodic sequence ofcontention-free sessions (CFS) in the transmission order of access point152 (AP1), access point 102 (AP2), and a third access point (AP3).

FIG. 9 shows a timing diagram illustrating the result of access point102 (AP2) retiring from the periodic sequence of contention-freesessions (CFS) shown in FIG. 8, which results in a gap of long enoughduration to inadvertently permit a DCF wireless station 104B to begincontention for the channel and transmit a packet that collides with theperiodic beacon packet of AP3.

FIG. 10 shows a timing diagram illustrating that when a periodicsequence of contention-free sessions (CFS) have intervals no longer thanPIFS separating them, only the first contention-free session (CFS) hasany probability of colliding with a DCF wireless station contending forthe channel.

DISCUSSION OF THE PREFERRED EMBODIMENT

The invention disclosed broadly relates to telecommunications methodsand more particularly relates to wireless cells that have overlappingstations contending for the same medium. An inter-cell contention-freeperiod value is assigned to a first access point station in the firstcell, associated with an accessing order in the medium for memberstations in the first and second cells. The access point in the firstcell transmits an initial shield packet to deter other stations fromcontending for the medium. The access point then transmits a beaconpacket containing the inter-cell contention-free period value to memberstations in the second cell. A second access point in the second cellcan then delay transmissions by member stations in the second cell untilafter the inter-cell contention-free period expires. The beacon packetsent by the first access point station also includes an intra-cellcontention-free period value, which causes the member stations in thefirst cell to delay accessing the medium until polled by the firstaccess point. After the expiration of the intra-cell contention-freeperiod, member stations in the first cell may contend for the mediumbased on the quality of service (QoS) data they are to transmit, usingthe Tiered Contention Multiple Access (TCMA) protocol.

Tiered Contention Multiple Access (TCMA) protocol is applied to wirelesscells which have overlapping access points contending for the samemedium. Quality of service (QoS) support is provided to overlappingaccess points to schedule transmission of different types of trafficbased on the service quality specifications of the access points. Adescription of Tiered Contention Multiple Access (TCMA) protocol appliedto overlapping wireless cells is provided in the following two copendingU.S. Patent Applications, which are incorporated herein by reference:Ser. No. 09/985,257, filed Nov. 2, 2001, by Mathilde Benveniste,entitled “Tiered Contention Multiple Access (TCMA): A Method ForPriority-Based Shared Channel Access;” and Ser. No. 10/187,132, filedJun. 28, 2002, by Mathilde Benveniste, entitled “Hybrid CoordinationFunction (HCF) Access Through Tiered Contention And Overlapped WirelessCell Mitigation.”

The method assigns a first scheduling tag to a first access pointstation in a first wireless LAN cell. The scheduling tag has a valuethat determines an accessing order for the cell in a transmission frame,with respect to the accessing order of other wireless cells. Thescheduling tag value is deterministically set. The scheduling tag valuecan be permanently assigned to the access point by its manufacturer, itcan be assigned by the network administrator at network startup; it canbe assigned by a global processor that coordinates a plurality ofwireless cells over a backbone network, it can be drawn from a pool ofpossible tag values during an initial handshake negotiation with otherwireless stations; or it can be cyclically permuted in real-time, on aframe-by-frame basis, from a pool of possible values, coordinating thatcyclic permutation with that of other access points in other wirelesscells.

An access point station 152 in wireless cell 150 is connected tobackbone network 160 in FIG. 1. The access point 152 signals thebeginning of an intra-cell contention-free session (CFS) of FIGS. 3 and4 for member stations 154A and 154B in its cell by transmitting a shieldpacket 118, during the period from T0 to T1. The shield packet 118 or119 is a short packet, such as a Physical Layer Convergence Procedure(PLCP) header without the MAC data, as shown in FIG. 2A. The shieldpacket 118 makes the wireless channel appear busy to any stationreceiving the shield packet. This includes not only the member stations154A and 154B in cell 150, but also any stations in another overlappedcell, such as cell 100. Access point 10 s and the stations 104A, 104B,and 106 of the overlapped cell 100 also receive the shield packet 118.All such stations listen to the channel and when they receive the shieldpacket 118, they defer transmitting on what they perceive to be a busychannel. The transmitting access point 152 is thus assured that no otherstation will begin contending for the medium while the access point 152is sending a beacon packet in the next step, shown in FIG. 1A. A timingdiagram for the transmission of the shield packet to begin theintra-cell contention-free session (CFS) is shown in FIGS. 3 and 4.

FIG. 2A shows the IEEE 802.11 packet structure 360 for a shield packet118. The shield packet structure 360 includes fields 361 to 367. Field365 is the PLCP header and field 367 is the empty frame body.

FIG. 1A shows the access point 152 of cell 150 transmitting the beaconpacket 124 during the period from T1 to T2. The beacon packet 124, shownin FIG. 2B, includes two contention-free period values. The first is theNetwork Allocation Vector (NAV) (or alternately its incremental valueΔNAV), which specifies a period value P3 for the intra-cellcontention-free period (CFP) for member stations in its own cell 150.The intra-cell contention-free period (CFP) is the duration of thecontention-free session (CFS) shown in FIG. 4. Member stations withinthe cell 150 must wait for the period P3 before beginning the TieredContention Multiple Access (TCMA) procedure. The other contention-freeperiod value included in the beacon packet 124 is the Inter-BSS NetworkAllocation Vector (IBNAV), which specifies the contention-free timeresponse (CFTR) period P4. The contention-free time response (CFTR)period P4 gives notice to any other cell receiving the beacon packet,such as cell 100, that the first cell 150 has seized the medium for theperiod of time represented by the value P4. A timing diagram for thetransmission of the beacon packet is shown in FIG. 4.

The beacon packet 124 is received by the member stations 154A (with alow QoS requirement 164A) and 154B (with a high QoS requirement 164B) inthe cell 150 during the period from T1 to T2. The member stations 154Aand 154B store the value of ΔNAV=P3 and begin counting down that valueduring the contention free period of the cell 150. The duration of theintra-cell contention-free period ΔNAV=P3 is deterministically set. Themember stations in the cell store the intra-cell contention-free periodvalue P3 as the Network Allocation Vector (NAV). Each member station inthe cell 150 decrements the value of the NAV in a manner similar toother backoff time values, during which it will delay accessing themedium. FIG. 2B shows the IEEE 802.11 packet structure 260 for thebeacon packet 124 or 120, including the increment to the NAV period andthe CFTR period. The value P4 specifies the Inter-BSS Network AllocationVector (IBNAV), i.e., the contention-free time response (CFTR) periodthat the second access point 102 must wait, while the first cell 150 hasseized the medium. The beacon packet structure 260 includes fields 261to 267. Field 267 specifies the ΔNAV value of P3 and the CFTR value ofP4. The method assigns to the first access point station a firstinter-cell contention-free period value, which gives notice to any othercell receiving the beacon packet that the first cell has seized themedium for the period of time represented by the value. The inter-cellcontention-free period value is deterministically set.

If the cells 100 and 150 are mostly overlapped, as in region 170 shownin FIG. 1A, then transmissions from any one station in one cell 150 willbe received by most or all stations in the overlapped cell 100. Thebeacon packet 124 transmitted by the access point 152 in cell 150 isreceived by all of the stations in cell 150 and all of the stations incell 100, in FIG. 1A.

Alternately, if only one or a small portion of stations are in theregion of overlap 170, then a contention-free time response (CFTR)packet will be used to relay the information in the beacon packet tothose stations remote from the transmitting station. The description ofthe CFTR packet and its operation is provided in the copending U.S.patent application Ser. No. 10/187,132, filed Jun. 28, 2002, by MathildeBenveniste, entitled “Hybrid Coordination Function (HCF) Access ThroughTiered Contention And Overlapped Wireless Cell Mitigation,” incorporatedherein by reference. For a partially overlapped region 170, any stationreceiving the beacon packet 124 immediately rebroadcasts acontention-free time response (CFTR) packet containing a copy of thefirst inter-cell contention-free period value P4. The value P4 specifiesthe Inter-BSS Network Allocation Vector (IBNAV), i.e., thecontention-free time response (CFTR) period that the second access point102 must wait while the first cell 150 has seized the medium. In thismanner, the notice is distributed to the second access point station 102in the overlapping, second cell 100.

FIG. 1B shows the point coordinator in access point 152 of cell 150controlling the contention-free period within the cell 150 by using thepolling packet “D1” 128 during the period from T2 to T3. A timingdiagram for the transmission of the polling packet is shown in FIG. 4.In the mean time, the second access point 102 in the second cell 100connected to backbone network 110 stores the first inter-cellcontention-free period value P4 received in the CFTR packet 126, whichit stores as the inter-BSS Network Allocation Vector (IBNAV). The secondaccess point 102 decrements the value of IBNAV in a manner similar toother backoff time values, during which it will delay accessing themedium. FIG. 1C shows the wireless station 154A in cell 150 respondingto the polling packet 128 by returning a responsive data packet “U1”140. A timing diagram for the transmission of the responsive data packet“U1” is shown in FIG. 4. Subsequent, similar exchanges in cell 150 areshown in FIG. 4, where access point 152 sends the polling packet “D2”and the polled station in cell 150 responds with data packet “U2”.Access point 152 then sends the polling packet “D3”, but there is noresponse from the polled station in cell 150; so within a PIFS interval,access point 152 sends the polling packet “D4” and the polled station incell 150 responds with data packet “U4”. It is seen at this point inFIG. 1D and FIG. 4 that the NAV value has been counted down to zero inthe stations of cell 150, signifying the end of the contention-freesession (CFS) for cell 150. FIG. 1D also shows that the IBNAV value inthe access point 102 and the CFTR value in the other stations of theoverlapped cell 100 have also been counted down to zero. The secondaccess point 102 in the cell 100 takes this as its cue to transmit ashield packet 119 to begin a contention-free session (CFS) for cell 100.

The method similarly assigns to the second access point 102 station inthe second wireless LAN cell 100 that overlaps the first cell 150 asecond contention-free period value CFTR=P7 longer than the firstcontention-free period value CFTR=P4. FIG. 1D shows the second accesspoint 102 in the cell 100 transmitting a shield packet 119 during theperiod from T4 to T5. The shield packet 119 is a short packet, such as aPhysical Layer Convergence Procedure (PLCP) header without the MAC data,as shown in FIG. 2A. The shield packet 119 makes the wireless channelappear busy to any station receiving the shield packet. This includesnot only the member stations 104A, 104B, and 106 in cell 100, but alsoany stations in another overlapped cell, such as cell 150. The accesspoint 152 and stations 154A and 154B of the overlapped cell 150 alsoreceive the shield packet 119. All such stations receiving the shieldpacket 119 delay transmitting on what they perceive to be a busychannel. The transmitting access point 102 is thus assured that no otherstation will begin contending for the medium while the access point 102is sending a beacon packet in the next step, shown in FIG. 1E.

Access point 102 in cell 100 sends its beacon packet 120 in FIG. 1E,including its contention-free period values of NAV (P6) and IBNAV (P7),to the member stations 104A (with a low QoS requirement 114A), 104B(with a high QoS requirement 114B) and 106 in the cell 100 during theperiod from T5 to T6. The stations 152, 154A, and 154B of the overlappedcell 150 also receive the beacon packet 120. FIG. 1F shows the pointcoordinator in access point 102 of cell 100 controlling thecontention-free period within cell 100 using the polling packet 132during the period from T6 to T7. FIG. 1G shows the wireless station 104Bin cell 100 responding to the polling packet 132 by returning: aresponsive data packet 142. It is seen at this point in FIG. 1H that theNAV value has been counted down to zero in the stations of cell 100,signifying the end of the contention-free session (CFS) for cell 100.FIG. 1H also shows that the IBNAV value in the access point 152 and theCFTR value in the other stations of the overlapped cell 150 have alsobeen counted down to zero. All of the stations in both cells 100 and 150have their NAV and CFTR/IBNAV values at zero, and they take this astheir cue to begin the contention period.

The method uses the Tiered Contention Multiple Access (TCMA) protocol toassign to first member stations in the first cell 150 a first shorterbackoff value for high Quality of Service (QoS) data and a first longerbackoff value for lower QoS data. FIG. 1H shows the station 154B in thecell 150, having a high QoS requirement 164B, decreasing its High QoSbackoff period to zero and beginning TCMA contention. Station 154Btransmits a request to send (RTS) packet 144 to station 154A during theperiod from T8 to T9. Station 154A responds by sending a clear-to-send(CTS) packet to station 154B.

Then, station 54B transmits its high QoS data packet 130 during theperiod from T9 to T10 in FIG. 1I. The backoff time is the interval thata member station waits after the expiration of the contention-freeperiod P3, before the member station 154B contends for access to themedium. Since more than one member station in a cell may be competingfor access, the actual backoff time for a particular station can beselected as one of several possible values. In one embodiment, theactual backoff time for each particular station is deterministicallyset, so as to reduce the length of idle periods. In another embodiment,the actual backoff time for each particular station is randomly drawnfrom a range of possible values between a minimum delay interval to amaximum delay interval. The range of possible backoff time values is acontention window. The backoff values assigned to a cell may be in thefor of a specified contention window. High QoS data is typicallyisochronous data such as streaming video or audio data that must arriveat its destination at regular intervals. Low QoS data is typically filetransfer data and email, which can be delayed in its delivery and yetstill be acceptable. The Tiered Contention Multiple Access (TCMA)protocol coordinates the transmission of packets within a cell, so as togive preference to high QoS data over low QoS data, to insure that therequired quality of service is maintained for each type of data.

The method uses the Tiered Contention Multiple Access (TCMA) protocol toassign to second member stations in the second cell 100 a second shorterbackoff value for high QoS data and a second longer backoff value forlower QoS data.

The first and second cells are considered to be overlapped when one ormore stations in the first cell can inadvertently receive packets frommember stations or the access point, of the other cell. The inventionreduces the interference between the overlapped cells by coordinatingthe timing of their respective transmissions, while maintaining the TCMAprotocol's preference for the transmission of high QoS data over low QoSdata in each respective cell.

FIG. 3 shows a timing diagram for the transmission of the shield packet.A CFS is started with the shield packet, which is a short frame (e.g.,Physical Layer Convergence Procedure (PLCP) header without MAC data).The AP will wait for an idle period of PIFS to transmit following theshield. If an (E)DCF transmission collides with the shield, the AP willhear the transmission and defer initiation of the CFS body. Aftercompletion of the (E)DCF transmission, the CFS will start, following aPIFS idle. Transmission of the shield before the CFS body is not alwaysneeded. For example, it is not needed if the AP knows that the idle gapbetween the CFS and the previous transmission is equal to PIFS; i.e.,when the backoff delay is 1, during the last busy period.

FIG. 4 shows a timing diagram of a sample CFS structure. It includes theshield packet, the beacon packet, and the exchange of data packetsduring the contention-free period shown in FIGS. 1, 1A through 1C.

All stations listen to the channel; and when they receive the shieldpacket, they defer transmitting on what they perceive to be a busychannel. The transmitting access point is thus assured that no otherstation will begin contending for the medium while the access point issending a beacon packet. If another station and the access point havesimultaneously begun transmission, then the benefit of the shield packetis that the other station's (E)DCF transmissions colliding with theshield packet will cause postponement of the start of the CFS body bythe access point until the channel is clear. The CFS is thus assured ofno (E)DCF conflict because of its shorter Arbitration Interframe Space(AIFS). While the other station's colliding (E)DCF transmission isunsuccessful, the CFS body will be transmitted later by the access pointwithout conflict. Channel time is saved this way if CFSs are longer thanDCF transmissions. This method can also be applied to PCFSs if there isno other mechanism to protect them from collisions with (E)DCFtransmissions, as there is in the point coordination function (PCF).Still further, a special shield packet may also be used in Inter-BSS NAVprotection.

The following definitions are believed to be helpful to an understandingof the invention.

Contention-free burst (CFB): A technique for reducing MAC layer wirelessmedium (WM) access overhead and susceptibility to collisions, in which asingle station may transfer a plurality of MAC protocol data units(MPDUs) during a single transmission opportunity (TXOP), retainingcontrol of the WM by using interframe spaces sufficiently short that theentire burst appears to be a single instance of WM activity tocontending stations.

Contention-free session (CFS): Any frame exchange sequence that mayoccur without contention following a successful channel access attempt.A CFS may involve one or more stations. A CFS may be initiated by anystation. A contention-free burst (CFB) and an RTS/CTS exchange are bothexamples of a CFS. A contention free burst (CFB) is a special case of acontention-free session (CFS) that is started by a hybrid coordinator(HC).

Contention-free period (CFP): A time period during operation of a basicservice set (BSS) when a point coordination function (PCF) or hybridcoordination function (HCF) is used and transmission opportunities(TXOPs) are assigned to stations by a point coordinator (PC) or hybridcoordinator (HC), allowing frame exchanges to occur withoutinter-station contention for the wireless medium (WM) and at regulartime intervals. The contention-free period (CFP) is the duration of acontention-free session (CFS).

Periodic contention-free session (PCFS): A contention-free session (CFS)that most occur at regular time intervals. A contention-free period(CFP) is an example of a PCFS. Both PCFSs and CFSs are needed, the PCFSsused for periodic traffic and the CFSs providing efficient use ofchannel time, as channel availability permits. When restricting the timeto the next access attempt, the channel cannot be used sooner, even ifneeded and available; it limits efficiency of channel re-use.

The description of the invention can be simplified by considering thatCFSs/PCFSs are initiated by Access Points (AP), However, CFSs/PCFSs canbe initiated by any station, whether or not it is an AP.

In a multi-BSS system, there will still be interference between BSSsassigned the same channel, once channels have been assigned to the BSSs.Allocation of the channel time is achieved through dynamic bandwidthallocation, which enables sharing of the channel time among co-channelBSSs efficiently, so that no channel time is left idle. Because there isno central controller, distributed, prioritized, dynamic bandwidthallocation algorithms are needed in order to coordinate multi-BSSchannel reuse.

A contention-free burst (CFB) is a special case of a contention-freesession (CFS) that is started by a hybrid coordinator (HC). It would bedesirable that CFSs/PCFSs have priority access over (E)DCFtransmissions. It would also be desirable for (E)DCF transmissions toaccess the channel at an assigned priority. It would still further bedesirable for CFSs to be able to regain control of the channelperiodically and conflict-free. It would also be desirable that thereare no conflicts with CFSs from other BSSs or (E)DCF transmissions.Finally, it would be desirable to have efficient channel re-use (with nochannel left idle) that is greater than or equal to the dynamicbandwidth allocation.

Other workers in the field have proposed that (E)DCF can be used by theCFSs to access the channel. The CFSs would be placed in the highestpriority class which is above the highest priority in (E)DCF. ShorterAIFS would be used for CFS access, which helps avoid collisions with(E)DCF transmissions. Backoff would help deal with CFS conflicts amongBSSs. However, such proposals raise concerns about random backoff. Witha random backoff, (E)STAs may access the channel before the HC, since(PIFS+1)=DIFS allows a (E)DCF station to transmit. Also, a long backoffleaves many idle slots, which would allow (E)STAs to transmit beforeHCs. Furthermore, a short backoff causes collisions between CFSs. Suchproposals also raise Concerns about fixed or minimum re-visit time. Whenrestricting the time to the next access attempt, the channel cannot beused sooner, even if needed and available. Also, although this may be agood approach for PCFSs, it limits efficiency of channel re-use.

These problems are avoided by the cyclic prioritized multiple access(CPMA) method for contention-free sessions (CFS). The cyclic prioritizedmultiple access (CPMA) method includes three features:

1—Fixed Deterministic Post-Backoff, which reduces conflicts between APs.

2—Staggered Start-up, which provides contiguous sequences of CFSs todeter collisions with (E)STAs. (Staggered Start-up is an optionalfeature.)

3—Preemptive ‘Pegging’, which preserves CFS sequence contiguity.(‘Pegging’ is an optional feature.)

The cyclic prioritized multiple access (CPMA) method requires amechanism for ‘busy’ channel detection (detection of the start and endof a CFS), such as the Inter-BBS NAV. This is described in the copendingU.S. patent application Ser. No. 10/187,132, filed Jun. 28, 2002, byMathilde Benveniste, entitled “Hybrid Coordination Function (HCF) AccessThrough Tiered Contention And Overlapped Wireless Cell Mitigation,”which is incorporated by reference. The cyclic prioritized multipleaccess (CPMA) method also requires fully overlapped BSSs or partiallyoverlapping BSSs with IBNAV protection. This is also described in thecopending U.S. patent application Ser. No. 10/187,132. An alternative tosuch protection is ‘parallel’ backoff to avoid “Neighborhood Capture”,as is described in the copending regular U.S. patent application Ser.No. 10/256,384, filed Sep. 27, 2002, by Mathilde Benveniste, entitled“Wireless LANS And Neighborhood Capture,” which is incorporated byreference.

The Fixed Deterministic Post-Backoff feature of cyclic prioritizedmultiple access (CPMA) reduces conflicts between access points ofoverlapping cells. Contention-free sessions (CFSs) can be generated, onefrom each overlapping cell. Each active access point engages in a fixeddeterministic post-backoff. A fixed deterministic backoff delay (Bkofftimes a fixed number of idle time slots) is used by all access points,with the value of Bkoff being greater than the number of overlappingcells. The Bkoff should large enough to enable the traffic that needs tobe accommodated by the channel. Each access point has a backoff timerthat is counted down using the shortest interframe space possible(typically PIFS). A contention-free session (CFS) is initiated when thebackoff timer expires, and it is then reset to the value of Bkoff tostart a new cycle. A cycle is measured in terms of idle time slots,instead of a fixed time interval. Contention-based transmissions can beattempted by an access point or other stations in the cell using theirassigned priority while the access point is counting down its backofftimer. A new access point can get started and resolve possiblecollisions by a small random backoff. Subsequent contention-freesessions (CFSs) will not conflict, given an existing sequence ofnon-conflicting CFSs, since the follower access point's backoff delayexceeds that of the leader's by at least one times the fixed number ofidle time slots. In this manner, contention-free sessions can beconducted without interference in the first and second cells.

FIG. 5 shows a timing diagram of non-conflicting contention freesessions (CFS) for access point 152 (AP1) and access point 102 (AP2).The figure shows the CFSs repeating in cycles. Contention-free sessions(CFSs) are generated, one from each overlapping BSS. Each active APengages in fixed deterministic post-backoff, which is characterized bythe post-backoff being ON. A fixed deterministic backoff delay, Bkoff,is used by all APs, with Bkoff greater than the number of overlappingBSS. The Bkoff should large enough to enable the traffic that can beaccommodated by the channel. FIG. 5 shows that the channel is accessedand the backoff timer is counted down using the shortest AIFS possible.A CFS is initiated when backoff expires and the backoff is reset toBkoff, which starts a new cycle. A cycle is measured in terms of idletime slots; it does not: represent a fixed time interval. (E)DCFtransmissions are attempted by their assigned priority while the AP iscounting its backoff down. A new AP can get started and resolve possiblecollisions by a small random backoff. FIG. 5 shows that subsequent CFSswill not conflict, given a sequence of non-conflicting CFSs. Becausetheir previous CFSs did not conflict, the follower AP's backoff delayexceeds that of the leader's by at least one.

In general, the cyclic prioritized multiple access (CDMA) method can getstarted by a random backoff, 0, 1 . . . [small value]. To reduce theprobability of collisions with (E)DCF transmissions, a contiguoussequence of CFBs (consecutive CFBs separated by idle gaps <=PIFS) isgenerated by observing the following startup procedure. If there is noother AP present, then the first AP will get started after waiting for acycle, which is the time it takes Bkoff idle time slots to expire (withdeferral time).

FIG. 6 shows a timing diagram of how access point 102 (AP2) listens fora PIFS idle following a busy channel and then starts transmitting aminimal interval after the contention free session (CFS) for accesspoint 152 (AP1). If one AP is operating and a second AP powers on, AP2listens to the channel until a PIFS Idle is observed following a busychannel, or for another indication of a CFS. Then it looks for the firstidle longer than PIFS and sets its post-backoff delay to transmit alwaysright after AP1. An idle period X=PIFS+x, x>0, has been detected at timet; AP2's backoff at time t is set at Bkoff−x. If several APs power onduring the same cycle, collision between new APs is possible. It can beresolved by a random backoff, 0, 11 . . . [small value]. As is shown inFIG. 6, AP2 listens to the channel until a PIFS Idle is observedfollowing a busy channel. Then it looks for the first idle longer thanPIPS and sets its post-backoff delay to transmit always right after AP1.When an idle period DIFS=PIFS+1 has been detected at time t, AP2'sbackoff at time t is set at Bkoff−1.

FIG. 7 shows a timing diagram of the successful startup of access point102 (AP2) after the contention-free session (CFS) for access point 152(AP1). These are non-conflicting, contiguous CFSs. Subsequent CFSs willbe contiguous, given a sequence of contiguous CFSs. Because theirprevious CFSs were contiguous, the follower AP's backoff delay exceedsthat of the leader's by exactly one. NAV protection and longer AIFSprevent DCF transmissions from conflicting with new CFSs.

FIG. 8 shows a timing diagram of access point 102 (AP2) transmitting apreemptive peg packet when it has no data to transmit in order tomaintain contiguity of its timing position in the periodic sequence ofcontention-free sessions (CFS) in the transmission order of access point152 (AP1), access point 102 (AP2), and a third access point (AP3).Pegging maintains contiguity. If an AP has no traffic, it will transmita short pegging packet and set its backoff=Bkoff. In this manner, nogaps of length DIFS+1 are left idle, and thus (E)DCF stations cannotseize the channel until all APs have completed one CFS per cycle.

FIG. 9 shows a timing diagram illustrating the result of access point102 (AP2) retiring from the periodic sequence of contention-freesessions (CFS) shown in FIG. 8, which results in a gap of long enoughduration to inadvertently permit a DCF wireless station 104B to begincontention for the channel and transmit a packet that collides with theperiodic beacon packet of AP3. If an AP retires or does not use pegging,gaps or idle periods >PIFS will occur. Then, a collision between itsfollower AP and an (E)DCF station is possible. Newly activated APs canhelp take up excess void (two retirements back-to-back) in the sequence.CFSs can be protected from (E)DCF transmissions by using a shieldpacket, as described in the copending U.S. patent application Ser. No.10/256,299 filed Sep. 27, 2002 by Mathilde Benveniste, entitled “Shield:Protecting High Priority Channel Access Attempts In Overlapped WirelessCells,” which is incorporated by reference.

FIG. 10 shows a timing diagram illustrating that when a periodicsequence of contention-free sessions (CFS) have intervals no longer thanPIFS separating them, only the first contention-free session (CFS) hasany probability of colliding with a DCF wireless station contending forthe channel. Given a contiguous sequence of CFSs (no gaps due toretirements), only the first AP can collide with (E)DCF stations. Inlighter traffic this is a low probability, Subsequent APs do not collidewith (E)DCF stations because of their shorter AIFS. CFSs can beprotected from (E)DCF transmissions by using a shield packet. If an APexperiences a collision with (E)DCF transmission, it may reset itsbackoff=Bkoff−x, thus moving from the head of the sequence behindanother AP. Another AP will thus take the lead position, sharing thecollision probability.

In summary, the cyclic prioritized multiple access (CPMA) methodincludes the fixed deterministic post-backoff feature, which preventscollisions among the different APs. The staggered start up featureachieves contiguous CFS sequences. Contiguity decreases the probabilityof collision with (E)DCF transmissions, since no idle gaps are left ofthe size of the AIFS of (E)DCF stations. The preemptive ‘pegging’feature maintains contiguity of CFS sequences. If collisions occur, asmall random backoff is used to resolve conflicts. The probability ofconflicts is still less than with simple random backoff.

PCFSs provide regular access to the channel for periodic traffic. Theuse of PCFSs alone cannot not provide efficient dynamic bandwidthallocation. CFSs generated on a contention basis must complement PCFSs.PCFSs and CFSs access the channel with the shortest AIFS. Quality ofservice (QoS) can be managed while using the cyclic prioritized multipleaccess (CPMA) method by an AP scheduling traffic as follows: Periodictraffic is transmitted in PCFSs; non-periodic traffic is placed eitherin a PCFS or in its allotted CFSs according to traffic priority;delay-sensitive traffic is scheduled first, followed by traffic of lowerpriorities.

Various illustrative examples of the invention have been described indetail. In addition, however, many modifications and changes can be madeto these examples without departing from the nature and spirit of theinvention.

What is claimed is:
 1. A method for reducing interference betweenoverlapping first and second wireless local area network cells in amedium, each of the first and second wireless local area network cellsincluding a respective plurality of member stations, comprising:coordinating, by a processor of a first access point station in thefirst wireless local area network cell, a periodic sequence of firstcontention-free sessions, each of the first contention-free sessionsincluding multiple bursts with other member stations in the firstwireless local area network cell, and retaining a control of the mediumby the first access point station by using interframe spaces between themultiple bursts such that the multiple bursts appear to contendingstations to be a single instance of activity in the medium; transmittingby the processor of the first access point station a preemptive pegpacket to maintain a contiguity of a transmission timing position of thefirst access point station with respect to a timing position of a secondcontention-free session transmitted by a second access point station;and wherein the preemptive peg packet is for preventing gaps to be leftidle for longer than a distributed coordination function interframespace; whereby other stations are prevented from using the distributedcoordination function interframe space to seize the medium, until thefirst and second access point stations have completed onecontention-free session per periodic cycle.
 2. The method of claim 1,further comprising: setting by the first access point station a backofftimer to a fixed deterministic post-backoff delay, which has a value ofBkoff times a fixed number of idle time slots, the value of Bkoff beinggreater than a number of overlapping cells; counting down the backofftimer by the first access point station; transmitting another one of thefirst contention-free sessions by the first access point station whenthe backoff timer expires; and resetting the backoff timer to the valueof Bkoff to start a new cycle.
 3. The method of claim 2, furthercomprising: the backoff timer being counted down using an interframespace.
 4. The method of claim 3, wherein the interframe space being apriority interframe space.
 5. The method of claim 3, wherein theinterframe space being a minimum arbitration interframe space.
 6. Themethod of claim 2, further comprising: listening by a processor of thesecond access point station in the second wireless local area networkcell to the activity in the medium and detecting an end to one of thefirst contention-free sessions indicated by an interval longer than apriority interframe space idle interval following an end to the activityin the medium; setting a second post-backoff delay by the processor ofthe second access point station to transmit a minimal interval after theone of first contention-free sessions of the first access point station;and coordinating by the processor of the second access point station inthe second wireless local area network cell a second contention-freesession of the second contention-free sessions, the secondcontention-free session including multiple bursts with other accesspoint stations in the second cell, and retaining the control of themedium by the second access point station by using interframe spacesbetween the multiple bursts such that the multiple bursts appear tocontending stations to be a single instance of activity in the medium.7. The method of claim 6, wherein the second post-backoff delay being aminimum arbitration interframe space.
 8. The method of claim 6, furthercomprising: initiating by the second access point station another one ofthe second contention-free sessions when the second post-backoff delayis counted down to zero; and resetting the second post-backoff delay tostart a new cycle.
 9. The method of claim 6, further comprising:separating consecutive ones of the first and second contention-freesessions by idle gaps which are less than a priority interframe space.10. The method of claim 9, where the setting the second post-backoffdelay further comprising: detecting an idle period X=priority interframespace +x, x>0 at time t; and setting the second post-backoff delay attime t, to a value of Bkof−x.
 11. A wireless communications systemhaving reduced interference between overlapping first and secondwireless local area network cells in a medium, each of the first andsecond wireless local area network cells including a respectiveplurality of member stations, comprising: a first access point stationcomprising a processor in the first wireless local area network cell,wherein the first access point station coordinates in the first wirelesslocal area network cell a periodic sequence of first contention-freesessions, each of the first contention-free sessions including multiplebursts with other member stations in the first wireless local areanetwork cell, and retaining a control of the medium by the first accesspoint station by using interframe spaces between the multiple burstssuch that the multiple bursts appear to contending stations to be asingle instance of activity in the medium; wherein the first accesspoint station transmits a preemptive peg packet to maintain a contiguityof a transmission timing position of the first access point station withrespect to a timing position of a second contention-free sessiontransmitted by a second access point station comprising a processor; andwherein the preemptive peg packet is for preventing gaps to be left idlefor longer than a distributed coordination function interframe space;whereby other stations are prevented from using the distributedcoordination function interframe space to seize the medium, until thefirst and second access point stations have completed onecontention-free session per periodic cycle.
 12. The system of claim 11,further comprising: the first access point station setting a backofftimer to a fixed deterministic post-backoff delay, which has a value ofBkoff times a fixed number of idle time slots, the value of Bkoff beinggreater than a number of overlapping cells; the first access pointstation counting down the backoff timer; the first access point stationtransmitting another one of the first contention-free sessions when thebackoff timer expires; and the first access point station resetting thebackoff timer to the value of Bkoff to start a new cycle.
 13. The systemof claim 12, further comprising: the backoff timer being counted downusing an interframe space.
 14. The system of claim 13, wherein theinterframe space being a priority interframe space.
 15. The system ofclaim 13, wherein the interframe space being a minimum arbitrationinterframe space.
 16. The system of claim 12, further comprising: thesecond access point station listening in the second wireless local areanetwork cell to the activity in the medium and detecting an end to oneof the first contention-free sessions indicated by an interval longerthan a priority interframe space idle interval following an end to theactivity in the medium; the second access point station setting a secondpost-backoff delay to transmit a minimal interval after the one of firstcontention-free sessions of the first access point station; and thesecond access point station coordinating in the second wireless localarea network cell a second contention-free session of the secondcontention-free sessions, the second contention-free session includingmultiple bursts with other access point stations in the second wirelesslocal area network cell, and retaining the control of the medium by thesecond access point station by using interframe spaces between themultiple bursts such that the multiple bursts appear to contendingstations to be a single instance of activity in the medium.
 17. Thesystem of claim 16, wherein the second post-backoff delay being aminimum arbitration interframe space.
 18. The system of claim 16,further comprising: the second access point station initiating anotherone of the second contention-free sessions when the second post-backoffdelay is counted down to zero; and the second access point stationresetting the second post-backoff delay to start a new cycle.
 19. Thesystem of claim 16, further comprising: the second access point stationseparating consecutive ones of the first and second contention-freesessions by idle gaps which are less than a priority interframe space.20. The system of claim 19, in which the setting the second post-backoffdelay further comprising: the second access point station detecting anidle period X=priority interframe space +x, x>0 at time t; and thesecond access point station setting the second post-backoff delay attime t, to a value of Bkoff−x.